This invention relates generally to location of a remote transmitter by received signal comparison from two correlated antennas and more particularly to the cell boundary determination and subsequent preliminary detection of when a handoff may be necessary in a multicell radiotelephone system utilizing comparison of signals received on correlated antennas having different antenna patterns.
Cellular mobile radiotelephone systems and other multiple radio coverage area systems which provide a continuous radio coverage of a geographic area from a plurality of non-colocated fixed sites characteristically employ an automatic transfer of radio communications capability from one fixed site to another as a remote transceiver unit travels from the radio coverage area of one fixed site to another. This handoff of communications has previously been described for the U.S. cellular radiotelephone service by Fluhr et al., "Control Architecture", Bell System Technical Journal, Vol. 58, No. 1, January 1979, pp. 43-69.
Variations in the overall cellular system design, direction of radio signal illumination, and techniques of cellular system growth have been the subject of several U.S. Pat. Nos., among which are: 3,663,762--Joel, Jr. --"Mobile Communication System"; 3,819,872--Hamrick --"Mobile Telephone Cellular Switching System"; 3,906,166--Cooper et al. --"Radio Telephone System"; 4,128,740--Graziano--"Antenna Array for a Cellular RF Communications System"; and 4,144,411--Frenkiel --"Cellular Radiotelephone System Structured for Flexible Use of Different Cell Sizes". Some of these traditional systems anticipate an increasing number of users with passing time and have developed graceful methods of subdividing and shrinking cell areas to enable multiple reuse of radio frequencies in a fixed geographical area. One of the limitations bounding the number of users in a cell or the smallest area to which a cell can be shrunk is that of the time required to determine the location of the remote unit and process a handoff. Generally, location and handoff need are determined from a periodic sampling of the signal quality received from each remote unit maintaining communications with a fixed station. Such a signal quality measurement may be made by comparison of the signal level above received noise as described in U.S. Pat. Nos. 4,549,311--McLaughlin--"Method and Apparatus for Measuring the Strength of a Radio Signal Frequency" and 4,704,734--Menich et al. --"Method and Apparatus for Signal Strength Measurement and Antenna Selection in Cellular Radiotelephone Systems". Additionally, techniques and equipment designed to enable handoff have been addressed in U.S. Pat. Nos. 4,485,486--Webb et al.--"Method and Apparatus for Assigning Duplex Radio Channels and Scanning Duplex Radio Channels Assigned to Mobile and Portable Radiotelephones in a Cellular Radiotelephone Communications System" and 4,696,027--Bonta--"Handoff Apparatus and Method with Interference Reduction for a Radio System".
However, when the density of the remote units becomes large or when the size of the cells becomes small or when the radio path is cluttered with areas of multipath interference and path obstructions, non-conventional handoff techniques must be employed to reduce the handoff decision time. One technique, addressed in U.S. patent application No. 4,797,947--Labedz --"Microcellular Communications System Using Macrodiversity", issued on Jan. 10, 1989 and assigned to the present assignee, utilizes digital radio systems technology and splits the handoff decision-making process between the fixed equipment and the remote units.
In urban areas and in locations with natural obstructions, it is common for radio signals from conventional cellular remote units to arrive at the fixed site receivers with reflections from nearby objects such as buildings or hills. It is also common for objects to obstruct the radio path from the remote units to the fixed site receivers. In either case, the received signal strength or quality can vary substantially and handoff determination based on signal quality can be subject to error. For example, a transmitting remote unit which moves to a location behind an obstructing building produces a signal strength at a fixed site receiver which decreases significantly, as though the remote unit had traveled beyond the boundary of the cell. A handoff would be initiated by the fixed site when it was unnecessary or improper to hand off the remote unit. A second handoff process would occur when the remote unit moves clear of the obstruction. It is as though the cell boundary were indistinct and holes of coverage could be found within the cell boundaries.
Thus, it is desirable that the boundary of a cell be given more distinct definition. Such definition would provide a reduction of handoff decision processing load in a cellular radiotelephone system.